The US military is investigating the killing of a wounded and apparently unarmed Iraqi prisoner inside a mosque during combat operations here, reports NBC.

NBC's Kevin Sites, who witnessed the incident Saturday, said Monday that the man was shot by a Marine who seemed unaware that the Iraqi was a wounded prisoner.

Bryan Whitman, a spokesman for the Defense Department, told NBC News that the military was investigating the incident observed by Sites. "We're confident it will be a thorough investigation," he said.

The incident occurred as a Marine battalion was taking part in a US-led counteroffensive intended to secure Fallujah.

According to Sites, 'the Marine battalion stormed an unidentified mosque Saturday in southern Fallujah after taking casualties from heavy sniper fire and attacks with rocket-propelled grenades. Ten insurgents were killed and five others were wounded in the mosque and an adjacent building,' NBC said.

'When the Marines left to advance farther south, the five wounded Iraqis, none of whose injuries appeared to be life-threatening, were left behind in the mosque for other Marines to evacuate for treatment.'

NBC said two units that were not involved in Friday's fighting advanced on the mosque, one moving around the back and the second, accompanied by Sites, from the front. Sites said he could hear gunfire from inside.

According to NBC, 'Sites was present when a lieutenant from one of the units asked a Marine what had happened inside the mosque. The Marine replied that there were people inside.'

"Did you shoot them?" the lieutenant asked.

"Roger that, sir," the second Marine replied.

"Were they armed?" the lieutenant asked.

The second Marine shrugged in reply.

NBC said Sites saw the five wounded men left behind on Friday still in the mosque. Four of them had been shot again, apparently by members of the squad that entered the mosque moments earlier.

'One appeared to be dead, and the three others were severely wounded. The fifth man was lying under a blanket, apparently not having been shot a second time.'

One of the Marines noticed that one of the severely wounded men was still breathing. He did not appear to be armed, Sites said.

According to NBC, 'the Marine could be heard insisting: "He's f---ing faking he's dead � he's faking he's f---ing dead." Sites then watched as the Marine raised his rifle and fired into the man's head from point-blank range.

"Well, he's dead now," another Marine said.

NBC said when told that the man he shot was a wounded prisoner, the Marine, who himself had been shot in the face the day before but had already returned to duty, told Sites: "I didn't know, sir. I didn't know."